Update travis config to deal with rubygems not supporting ruby < 2.3 any longer.

See ruby/rubygems#2534.

Author: gkellogg

.travis.yml

@@ -1,7 +1,9 @@
 language: ruby
 bundler_args: --without debug
 script: "bundle exec rspec spec"
-before_install: "gem update --system"
+before_install: 
+  - 'gem update --system --conservative || (gem i "rubygems-update:~>2.7" --no-document && update_rubygems)'
+  - 'gem update bundler --conservative'
 env:
   - CI=true
 rvm: